window.resize_increments miscalculates the window size for some values of window.padding

When both: (a) window.resize_increments = true, and ... (b) window.padding is greater than zero ... then the actual size of the window (in terms of rows and lines of text displayed) will be smaller than the requested size.

I have observed this problem with Alacrity version 0.16.1 on Ubuntu 26.04 with Xorg. I suspect the problem will occur regardless of what font is used. I have tested with two fonts. One font was a bit-mapped PCF font, and I believe the other font was a vector font.

Here is an alacritty.toml file that can reproduce this issue:

[window]
  dimensions  =  {  columns = 80,  lines = 24  }
  padding  =  { x = 2, y = 2 }
  resize_increments  =  true

When I run Alacritty with the above alacritty.toml file, my expectation is that the window will hold 80 columns and 24 lines of text.

However, the actual result will be a window that holds 79 columns and 23 lines of text.

I suspect that the following is happening:

1. Alacritty creates a window large enough for 2 padding and 80x24 characters.
2. Alacritty then applies the resize_increments constraint.
3. The resize_increments constraint ignores the padding, and consequently slightly shrinks the window.
4. The resulting window would be large enough for 80x24 if the padding were zero.
5. However, due to the positive padding, the resulting window is only large enough for 79x23 characters. The window also has unused gaps of space along the right and bottom edges.

The same problem occurs if I use the mouse to resize the window. I am running the Openbox window manager on Xorg on Linux.

1. I resize the window by dragging the mouse.
2. While I am resizing the window, Openbox displays the size of the window (in columns and lines) in a little popup box.
3. However, the actual size of the text grid inside the window will be smaller than the size that Openbox is displaying.

For your convenience, here is a simple bash script that will display the current size of the window.

while  true  ;  do
  clear
  echo
  echo  "    $COLUMNS x $LINES"
  sleep  0.1
  done